Heat Pump

ABSTRACT

A heat pump device in which a temperature difference is established between two heat exchangers by inducing cyclical expansion and compression pulses in a working fluid vapour or gas which passes through an adsorbent porous solid located between the heat exchangers.

This invention relates to a heat pump device particularly for air conditioning, refrigeration and heat pumping systems. The device relates especially to systems that contain no fluids known to have adverse effects on the stratospheric ozone layer or to have high global warming potentials relative to carbon dioxide. The device may provide a direct replacement for any apparatus that currently employs a mechanical vapour recompression or refrigerant/absorption solvent cooling or heat pumping system.

In this specification the term ‘heat pump’ refers to any powered device which moves heat from a source to a sink against a thermal gradient. A refrigerator is a particular type of heat pump where the lower temperature is required for an intended application. The term ‘heat pump’ may be also used in a more limited sense than in this specification to describe a powered device which moves heat from a source to a sink against a thermal gradient where the higher temperature is required. The distinction between a refrigerator and a narrowly-defined heat pump is merely one of intended purpose, not operating principle. Indeed, many air conditioning systems are designed to supply either heating or cooling depending upon the user's need at a specific time.

Chlorofluorocarbons (CFCs e.g. CFC 11, CFC 12) and hydrochlorofluorocarbons (HCFCs eg HCFC 22, HCFC 123) are stable, of low toxicity and non-flammability providing low hazard working conditions when used in refrigeration and air conditioning systems. If released they permeate into the stratosphere and attack the ozone layer which protects the environment from the damaging effects of ultraviolet rays. The Montreal Protocol, an international environmental agreement signed by over 160 countries, mandates the phase-out of CFCs and HCFCs according to an agreed timetable.

The CFCs and HCFCs have been superseded in new air conditioning, refrigeration and heat pump equipment by hydrofluorocarbons (HFCs eg HFC 134a, HFC 125, HFC 32) either as pure fluids or as blends. To accelerate the phase-out of CFC and HCFC existing units have also been retrofitted with appropriate HFC blends. Although HFCs do not deplete stratospheric ozone they are known to contribute to global warming. By the provisions of the Kyoto Agreement governments have undertaken to limit or cease the manufacture and release of these compounds. Some countries have already decided that phase-out of HFCs should commence sometime during the next decade and are actively promoting the development of non-halogen containing fluids.

The fluids in devices intended to replace HFC-containing units must have very low or preferably zero global warming potentials. Preferably they should be compounds that are found naturally and whose properties are well understood so that damage to the environment from anthropogenic releases can be avoided. Furthermore, devices should be at least as energy efficient as the HFC containing units they are replacing to ensure that their contributions to global warming due to fossil fuel power station emissions are no greater. Preferably the devices should have better energy efficiencies.

According to the present invention there is provided A heat pump device comprising:

-   -   at least one heat exchanger;     -   a body of porous adsorbent material the body having an inlet and         an outlet the body being disposed in thermal contact with the         heat exchanger,     -   means for passing a working fluid through the body, and     -   means for inducing cyclical compression and expansion pulses in         the working fluid, to cause the working fluid to flow from the         inlet to the outlet to create a temperature gradient in the body         between the inlet and outlet.

The invention provides a heat pump device in which a temperature difference is established by inducing cyclical expansion and compression pulses in a working fluid vapour or gas which passes through an adsorbent porous solid located in one or more heat exchangers.

The device may be specifically arranged so that heat is emitted at an elevated temperature at a first one location or end of the heat exchanger and taken in at a lower temperature at a second location or end of the heat exchanger.

In use of the device the working fluid enters the adsorbent solid at one end under elevated pressure and is removed at lower pressure or by suction at the second end.

The device may also include a heat transfer fluid.

In a preferred embodiment the device includes a heat transfer fluid, means for passing the heat transfer fluid in thermal contact with the heat exchanger, arranged so that the flow of heat transfer fluid removes or adds heat from or to the heat exchanger.

Preferably the direction of flow of the heat transfer fluid changes with the compression and expansion pulses of the working fluid.

More preferably the direction of flow of the heat transfer fluid reverses in coordination with the compression and expansion pulses of the working fluid.

In a particularly preferred embodiment the reversal of direction of flow of the heat transfer fluid is synchronized with said pulses.

The frequency of the cyclical motion of the working fluid may be the same as the frequency of cyclical motion of the heat transfer fluid.

The working fluid can be a vapour, gas or liquid preferably a vapour or gas. In the first part of the cycle the heat transfer fluid is caused to move in use from the low temperature end towards the higher temperature end of the heat exchanger to remove heat from the hot end of a heat exchanger and reject it into a suitable heat sink. In a second part of the cycle the heat transfer fluid, is caused to move in an opposite direction from the higher temperature end towards the lower temperature end of the heat exchanger where it is cooled before entering the refrigerated volume. To achieve a cyclical operation the heat transfer fluid may be caused to oscillate forwards and backwards along the heat exchanger with same frequency as the cyclical compression and expansion, that is pulsing, of the working fluid. When the working fluid is being admitted to the porous solid under compression the heat transfer fluid moves in the opposite direction to the working fluid. When the working fluid is being removed from the porous under suction the heat transfer fluid moves in the same direction as the working fluid.

The means for inducing pulses in the working fluid may be a positive displacement compressor.

Alternatively the means for inducing pulses in the working fluid includes a valve switching system and a compressor. Preferably the valve switching system alternately connects the body of adsorbent material to high and low pressure reservoirs of the working fluid.

Optionally but preferably the device comprises a further heat exchanger adapted to remove heat of compression from the working fluid vapour or gas before it contacts the adsorbent.

The hot and cold temperatures generated depend upon the specific applications for which the embodiments of this invention are used, in particular whether refrigeration or air conditioning is required. In this context air conditioning is to be understood to include both room cooling and heating. Devices that can provide both heating and cooling depending upon the requirements of the user are sometimes called reversible air conditioners.

Preferably the temperature gradient comprises a relatively high temperature at the inlet and a relatively low temperature at the outlet.

In an alternative embodiment the working fluid is a blend of a relatively strongly adsorbed fluid and a relatively weakly adsorbed fluid. These fluids do not strongly interact with each other. The more weakly adsorbed fluid may serve to sweep the more strongly adsorbed from the adsorbent during suction and carries it to the adsorbent during compression. Examples of such combinations are carbon dioxide/nitrogen and carbon dioxide/argon with activated porous carbon as the adsorbent. A preferred embodiment the blend is a combination of a relatively strongly adsorbed gas, especially ammonia or carbon dioxide with one or both of the light gases hydrogen and helium. The light gases have high thermal conductivies compared to the heavier working fluid gases or vapours and thus improve heat transfer to and from the adsorbent during the adsorption and desorption of the heavier working fluid. A combination of helium and carbon dioxide is especially preferred because the blend is non-flammable.

In a further embodiment of this invention a blend of two or more working fluids is selected such that one adsorbs more strongly than the other. When the heat pump is operating under a relatively light load the less strongly adsorbed fluid is at a higher concentration in the circulating fluid than its concentration in the blend originally introduced into the unit. Conversely the concentration of the more strongly adsorbed component in the mixture remaining on the adsorbent is greater than the original blend concentration. When the heat pump is operating under a heavy load the concentration of the circulating fluid contains a greater proportion of the more strongly adsorbed fluid and the composition of the circulating fluid approaches that of the loaded blend. By using a blend with these properties the heat pump can adapt its operation to changing loads and thus maximize its energy efficiency. An example of such a blend is a combination of propane and carbon dioxide used with a porous carbon adsorbent.

For single room air conditioning the heat transfer fluid will be generally be air. In cooling mode the cold temperature will be generally about 5 to about 15° C. while hot temperatures will be generally about 35 to about 60° C. Cooling powers will typically range from about 3 kW to about 100 kW. In heating mode the output temperature to the room will be typically be about 20 to about 30° C. and input temperatures from the outside air typically about 2 to about 15° C. Heating powers will typically be about 4 to about 150 kW. Some devices may be designed simply to provide heating and may generally be described as heat pumps, although this is a more restricted use of the term than that used in this specification.

For the air conditioning of large buildings with multiple rooms such as hotels and office blocks the heat transfer fluid can be water which maybe piped through each room where air will be blown over the cold water piping to provide the required cooling. This system is analogous to conventional chiller installations. The temperature of the water fed into the system will be typically at about 5-10° C. and the water returning to the device will be typically at about 10 to 15° C. In heating mode the temperature of the water leaving the device will be about 25 to about 40° C., while the return water will be typically about 15 to about 30° C. Cooling powers typically range from 50 kW to 10 MW. Chillers can also be used in process industries, for example cooling condenser water in distillation equipment.

In one embodiment of the invention the device is used to provide refrigeration typically at temperatures down to about −30° C. In this application it is preferable to use a relatively low freezing point heat transfer liquid. In a further embodiment of this invention equipment performance is optimised by carrying out the heat pumping process over two or more stages. This approach is especially advantageous for temperatures below about −20° C. Although carbon dioxide is a good refrigerant for Rankine cycle heat pumps with condenser temperatures lower than about 0° C., it has a critical temperature of 31° C. and a high critical pressure of 72 bar. For this reason it is unattractive for use in heat pumps where the heat output temperature is above 0° C.

An especially preferred embodiment comprises a conventional heat Rankine cycle stage wherein carbon dioxide is the working fluid and which pumps heat from a temperature in the range generally about −55 to about −10° C. and reject it at a temperature generally in the range about −20 to about 0° C. to the low temperature side of a second stage device employing the present invention. This second stage built according to this invention rejects heat at temperatures of about 35 to about 70° C. while operating at maximum pressures in the range about 10 to about 30 bar, typical of current HFC based refrigeration equipment.

The working fluid may be selected from any chemically stable fluid that can be reversibly adsorbed onto and desorbed from a suitable porous solid. Preferred fluids include carbon dioxide, air and nitrogen. The working fluid may be a fluorocarbon or a mixture of fluorocarbons boiling between −140° C. and 40° C., preferably between −90° C. and 0° C., more preferably between −90° C. and −20° C. CFCs, HCFCs and EFCs are acceptable in those territories where their use is permitted but are not preferred because of their adverse environmental effects. Preferred fluids are those that occur naturally. Hydrocarbons and hydrogen can be used in applications where flammability is not an issue. Ammonia is acceptable for applications where exposure to humans and animals can be prevented. For applications where a fluorinated fluid is preferred then HFCs, perfluoro-iodides and unsaturated fluorinated compounds containing 2 to 6 carbon atoms can be used with low global warming potentials relative to CO₂ preferably less than 150, more preferably less than 100 and most preferably less than 10. Preferred compounds are fluorinated olefins. More preferred are fluoroolefins containing a trifluorovinyl group. Even more preferred are fluoro-olefins containing at least one hydrogen atom. Especially preferred are fluoro-propenes and their blends. Where fluorinated compounds are not acceptable, especially preferred working fluids are CO₂ and N₂ which combine low environmental impact with low toxicity and non-flammability.

In the literature the term ‘porous solid’ is used for materials with a wide range of properties. Many solids have a very limited porosity including the protective oxide layers found on metals. In this specification the term, ‘porous solid’ is used to describe a material with a particular combination of properties.

Firstly, the internal surface area of a preferred porous solid is greater than about 10 m²g⁻¹, more preferably greater than about 100 m²g⁻¹, most preferably greater than about 1000 m²g⁻¹.

Secondly, the void space in a preferred porous solid is distributed between a combination of macro-, meso- and micro-pores. The porous solid has at least 10% of its void volume in the form of micropores with diameters less than about 2 nm, at least 5% of its void volume in the form of mesopores with diameters less than about 50 nm.

Thirdly a preferred porous solid is capable of reducing the pressure of the working fluid vapour or gas in contact with it, i.e. it adsorbs the working fluid.

Fourthly, the adsorption process must be reversible, e.g. it must be possible to desorb the working fluid by reducing its pressure or by raising the temperature of the porous solid.

Fifthly, a preferred porous solid must be capable of adsorbing the working fluid gas above its critical temperature.

A wide range of porous materials may be employed. Silica, for example fumed silica, granular silica or aerogel silica, including granular, monolith and flexible blanket aerogels may be used. Natural or artificial glasses, ceramics or molecular sieves may be used. Carbons which may be used include granules, monoliths, fabrics, aerogels and membranes. Examples of porous carbons suitable for this invention are described in PCT/GB01/04222 the disclosure of which is incorporated into the specification by reference. Various organic materials including resorcinol-formaldehyde foams or aerogels, polyurethane, polystyrene or other polymers in the form of foams and aerogels. Polymers of intrinsic porosity in which the tailored pore sizes are created by the 3-dimensional linking of appropriate precursor molecules with constrained geometries are also suitable for this invention. A range of composite materials are acceptable, including silica-carbon composites.

Porous materials may be made by blowing polymeric foams, and by sol-gel processes for manufacture of porous ceramics, silica or other mineral aerogels or organic aerogels. Organic materials for example coconut and coal, may be pyrolysed, and then further processed for example by treating with steam, to produce activated carbons. Polymer aerogels may be pyrolysed to produce carbon aerogels. Hydrocarbons may be pyrolysed to produce carbon membranes. Molecular sieves and carbon black or by plasma processes such as the APNEP (Atmospheric Pressure Non-Equilibrium Plasma) system developed by CTech Ltd. Carbon based materials, such as activated carbons derived from biomass precursors, e.g. coconut shell, are especially preferred since they are obtained from sustainable resources, require minimal energy input in their manufacture and effectively sequester atmospheric carbon dioxide as carbon within heat pump devices. At the end of the working life of a device such carbon adsorbents can be removed recovered and burnt recovering their energy content, originally captured when the biomass was formed, returning the carbon dioxide to the atmosphere. Since the gas originated from this source the combustion is CO₂-neutral. Preferably the carbon adsorbent would be buried in landfill, or in the subduction zones at boundaries of tectonic plates, or recycled to new equipment thus ensuring that the carbon is permanently removed from the atmosphere.

Inorganic porous materials may be obtained by thermolysis, for example production of fumed silica from silicon tetrachloride using an oxy-hydrogen flame or by plasma processes. Organic-inorganic precursors may be processed by thermolysis to produce molecular sieves. Natural mineral hydrates may be thermalised, for example vermiculite and perlite.

In one embodiment of this invention a heat pumping device comprises an adsorbent porous solid blend whose the properties vary between the high and low temperature ends of the adsorbent tube beds.

In a further embodiment of this invention the porous solid is selected such that its permeability to gas flow along the tube is sufficiently low to allow a pressure gradient to be generated along the tube during compression and during suction. The permeability of the adsorbent can be controlled in various ways. For example the range of particle sizes can be selected according to the length and cross-sectional area of the tube and the working fluid flow rate to give the desired pressure gradient. Alternatively the porous solid particles may be compressed into a monolith with a suitable binder to give the required pressure gradient.

The gas or vapour working fluid may be matched to preferred porous solids such as: carbon (e.g. graphite, activated carbon, charcoal, aerogel), silica (fumed, aerogel, alkylated aerogel), alumina, alumino-silicates (molecular sieves), and organic polymers (e.g. polystyrene, polyurethane, polyacrylate, polymethacrylate, polyamines, polyamides, celluloses), metal sponges (e.g. Ni, Ti, Fe), and metals or metal complexes supported on organic polymers or carbon.

Not all these gases and their combinations with the available porous solids may be appropriate. Although the CFCs and the HCFCs continue to be manufactured and used in the developing world their phase-out under the Montreal Protocol is already occurring. In territories when continued use of these chlorinated fluids is still legal then they can be used in combination with activated carbon, silica, or an organic polymer. SO₂ and HFCs can be used with carbon, silica, alumina or an organic polymer, especially those with “basic” atoms such as O and N or “acidic” H atoms. In territories where phase-out of HFCs is not currently being considered, then their use in the present invention is acceptable, but not preferred because their global warming potentials are much higher than some of the other gases listed above. SO₂ is not preferred because of its toxicity.

Hydrocarbons can be coupled with carbon, alkylated silica or an organic polymer, especially a hydrocarbon polymer such as polystyrene. Although more preferred than the halogenated fluids and SO₂, hydrocarbons are restricted to applications where the appropriate precautions can be taken against their marked flammability hazard, for example in large industrial applications or low-inventory, hermetically-sealed systems such as domestic refrigerators. A further disadvantage is that the enthalpy changes associated with the sorption/desorption of hydrocarbons is less than for more polar gases, notably CO₂, SO₂ and NH₃. In some territories hydrocarbons are also disliked because any leak to the atmosphere where exposure to sunlight generates “photo-chemical smog”.

Hydrogen is readily sorbed and desorbed from various metal alloys, notably those containing nickel. Hydrogen is preferred to hydrocarbons because it will interact more strongly with metals than hydrocarbons with the sorbents listed above. Like hydrocarbons hydrogen reacts with atmospheric hydroxyl radicals that play a key role in removing naturally-emitted hydrocarbons as well as man-made pollutants such as HFCs. Increased hydrogen emissions can thus indirectly increase globally warming.

Ammonia can be used with carbon, silica or with an organic polymer. It is suitable for applications where its toxicity and flammability can be controlled, for example large commercial and industrial applications or low inventory, hermetically sealed domestic applications.

The most preferred working fluid is carbon dioxide. Although carbon dioxide derived from fossil fuel is the single largest contributor to global warming the quantities required for this invention would be very small. By obtaining carbon dioxide from a natural source, such as biomass fermentation, any gas emitted from the device would have zero contribution to global warming. Carbon dioxide has low toxicity, is non-flammable and is readily adsorbed by a variety of porous solids including carbon, silica and organic polymers, especially those containing basic atoms such as O and particularly N. The ability of porous solids to adsorb CO₂ can be enhanced by impregnating the solids with compounds containing groups capable interacting with the fluid. Nitrogen and oxygen containing substances can be employed. Amines, amides alcohols, esters and ketones are preferred. More preferred are amines, amides, and urethanes with high boiling points, preferably above 100° C. Especially preferred are substances where molecular mass per N atom is less than 200, preferably less than 100 and most preferably less than 60. A particularly preferred substance is poly-ethyleneimine.

For very low temperature refrigeration involving temperatures below −55° C. carbon dioxide is not practical because its triple is −56.7° C. For sub −55° C. temperatures N₂ is preferred as a working fluid with adsorbent, such as activated carbon. The preferred heat transfer fluid is the atmosphere within refrigerated enclosure, which in many cases will be air. This design will provide cooling in the range −130° to −40° C. and will reject heat in the range −55 to −25° C. to a higher temperature stage.

In further embodiment of this invention blends of gases or vapours can be employed provided that they do not chemical react. Thus a hydrocarbon such as propane can be mixed with carbon dioxide.

Preferably the temperature changes generated when the fluid reversibly adsorbs and desorbs should be greater than 5° C. and more preferably greater than about 10° C.

Seven important parameters may contribute to the temperature change:

(a) the integrated heat of adsorption (IHA) measured between the lowest and highest pressure between which the adsorbent operates; (b) the heat capacity of the adsorbent (HCA); (c) the density of the adsorbent (DA), (d) the internal surface area of the adsorbent (SAA), (e) maximum operating pressure (MP), (f) the rates of adsorption/desorption and (g) the thermal conductivity of the adsorbent.

The integrated heat of adsorption (IHA) is a function the interaction of the fluid with the porous solid and is defined as the total heat generated when the fluid adsorbs onto the solid as its pressure is raised from a lower pressure to a higher pressure. The higher the IHA the stronger is the interaction of the fluid with solid. Preferably IHA should be at least 50 kJ/kg and more preferably greater than 100 kJ/kg. Most preferably the IHA, expressed in units of kJ/mol, should be comparable with the latent heats of condensation of existing refrigerants.

The higher the IHA the lower will be the pressure of the fluid above the adsorbent. A maximum working pressure just below approximately 2 bar at the heat rejection temperature is advantageous in that it keeps the pressure at any point in the device below the pressure at which pressure regulations apply. This allows the device to be manufactured more cheaply. It does require the use of high volume throughput compressor such as a centrifugal compressor and this is especially suited to large water chillers for example employed for air conditioning public buildings. An IHA which reduces the fluid pressure over the adsorbent significantly below 2 bar is not preferred since it increases the size of the components, notably the compressor, without any economic advantage.

In devices working with maximum operating pressures above approximately 2 bar the IHA is preferably chosen such that the pressure of the adsorbent at the lowest working pressure of the device is not less than approximately 1 bar to prevent the ingress of atmospheric gases which are not significantly adsorbed by the porous solid. Preferably the IHA is chosen such that in a given application the lowest operating pressure is not less than 1.5 bar.

A special advantage of the present invention is that it allows even relatively small temperature changes below 5° C. induced by fluid adsorption and desorption to generate the required substantial temperature differences between the ends of the adsorbent tubes, for example a difference of 35° which is required to generate the cold and hot air temperatures of 10° C. and 45° C. typically required for air conditioning applications. Despite this advantage larger temperature changes facilitate heat exchange between the bed and the external heat transfer fluid. Preferably changes on adsorption and desorption are greater than 5° C. and more preferably greater than 10° C. The higher the IHA the larger the temperature change obtained. Lower adsorbent heat capacities (HCA) also provide higher temperature changes. Preferably HCA is less than 2.00 kJ/kgK and more preferably less than 1 kJ/kgK and most preferably less than 0.8 kJ/kgK. Especially preferred are porous carbon materials and metal adsorbents for hydrogen with HCAs less than 0.75 kJ/kgK.

Although a group of adsorbents may have similar IHA their adsorption capacities (CA) for a working fluid will depend upon the numbers of active sites available per unit mass. The number of active sites tends to be related to the internal surface area of the porous solid (USA) accessible to the fluid molecules, thus the higher ISA the greater the capacity of the solid per unit mass to adsorb the fluid at a given pressure. ISAs of at least 1000 m³/g are most preferred.

Provided the IHAs, SHAs and ISAs for a series of adsorbents with a specified fluid are similar the temperature changes will be essentially independent of their densities (DA). But the temperature changes also depend upon the heat capacities of the materials from which the adsorbent tube is manufactured. The quantities of these materials can be minimised by selecting porous solids with high densities provided this does not affect the other adsorbent physical properties discussed above. Furthermore the quantity of the heat exchange fluid which removes heat from and adds heat to the adsorbent tube can also determine the temperature changes. Low inventories and high flow rates of the heat exchange fluid are preferred.

High maximum adsorption pressures maximise the capacity of the adsorbent for the working fluid. However as the pressure increases the incremental capacity of the adsorbent diminishes while the gauge of the pipe required to withstand the pressure increases, with a consequent increase in mass and hence thermal capacity of the heat exchanger. The latter reduces the magnitude of the temperature changes obtained on adsorption and desorption. The optimum maximum pressure depends upon the pressure/adsorption properties of the porous solid. For the combination of CO₂/activated carbon the optimum working pressure is generally around 20 bar.

The thermal conductivity of the adsorbent is important. Porous solids, especially in particulate or granular form, have low thermal conductivities consequently heat transfer during adsorption and desorption limits the cycle time of the beds. In a preferred embodiment the heat pump comprises one or more adsorbent tubes which are long in comparison to their width or diameter and are adapted for progressive removal or addition of heat from one end to the other end. The ratio of tube length to diameter should be preferably greater than about 5:1, more preferably greater than about 10:1 and most preferably greater than about 20:1.

To improve the thermal conductivities of adsorbents they can advantageously be composed partially or entirely from heat conducting materials. The latter may include graphite, preferably as flakes, fibres or foams; metal mesh, powder, wire or fibres, preferably comprising high thermal conductivity metals such as copper and aluminium; organic polymers with high thermal conductivities, such as polyaniline and poly-pyrrolidine or mixtures thereof. Such polymers, in at least one chemical form, generally have good thermal and electrical conductivities.

In a preferred embodiment of this invention such thermally conducting polymers containing basic nitrogen atoms and constituting at least a proportion of the porous solid are used. Such a porous solid may also contribute to the adsorption of carbon dioxide. Compressing the porous solid into a monolith also improves thermal conductivity.

Table 1 lists examples of various adsorbents and their thermal conductivities. This demonstrates that the addition of a heat conducting additive substantially improves the thermal conductivity of an adsorbent.

TABLE 1 Thermal Conductivity Adsorbent W/(m · K) Consolidated zeolite 13X 0.58 Consolidated zeolite + expanded graphite  5-15 Fused silica 1.3 Silica gel + 20-30% expanded graphite 10-20 Monolithic carbon 0.27-0.60 Granular carbon 0.1 Monolithic carbon + aluminium laminate 20

Preferred adsorbents have thermal conductivities greater than about 0.5 W/(m.K), more preferably greater than about 5 W/(m.K) and most preferably greater than about 50 W/(m.K).

The adsorbent heat exchanger configuration also influences the ease with which heat is transferred to and from the porous solid. An important requirement is to maximise the heat exchange without increasing the thermal capacity of the metal components of the heat exchanger so that temperature changes do not fall below the preferred value of 5° C.

The invention is further described by means of example but not in any limitative sense with reference to the accompanying drawings of which:

FIG. 1 is an illustration of a heat exchanger containing adsorbent with an outer duct containing heat transfer fluid;

FIG. 2 is a cross-section through the heat exchanger assembly shown in FIG. 1;

FIG. 3 is a cross-section through a multiple adsorbent tubes contained within a cylindrical heat transfer liquid duct;

FIG. 4 is a cross-section through multiple adsorbent tubes contained within an hexagonal heat transfer liquid duct;

FIG. 5 is a section of single adsorbent tubes showing longitudinal heat transfer fins and spiral heat transfer fins;

FIG. 6 is an illustration of an adsorbent pipe heat exchanger with an internal heat transfer fluid tube;

FIG. 7 is a cross-section of the heat exchanger illustrated in FIG. 6;

FIG. 8 is a cross-section through an adsorbent tube containing multiple heat transfer fluid tubes;

FIG. 9 illustrates a spiral wound heat adsorbent tube;

FIG. 10 is a cross-section a spiral wound heat exchanger tube enclosed within a duct.

FIG. 11 is a schematic view of a first device in accordance with the invention;

FIG. 12 is a schematic view of a second device in accordance with the invention;

FIGS. 13 a to 13 g are views of a third device in accordance with the invention; and

FIG. 14 is a schematic view of a fourth device in accordance with the invention.

FIGS. 1 and 2 illustrate a first embodiment of the invention.

In this embodiment the heat transfer fluid is a liquid constrained to flow in an external duct concentric to the adsorbent tube as shown in FIGS. 1 and 2. A heat transfer fluid duct 1 contains an adsorbent tube. The flow of working fluid through the adsorbent tube is controlled by valves 1.2 and 1.3. When adsorbent tube is under suction valve 1.6 is open and valve 1.7 is closed. The working fluid flows out at 1.6 as shown in the diagram. Simultaneously heat transfer fluid flows into the duct at 1.4 and out at 1.5. When the unit is under compression 1.6 is closed and 1.7 is open. The heat transfer liquid flows in the reverse direction, i.e. in at 1.5 and out at 1.4. In FIG. 2 adsorbent 2.1 containing tube 2.4, the heat transfer liquid 2.2 and the heat transfer liquid duct 2.3 are shown. For a larger device a multiplicity of absorber tubes can be contained within a single liquid duct as shown in FIGS. 3 and 4. 3.1 and 4.1 are the adsorbents, 3.2 and 4.2 the heat transfer fluids, 3.3 and 4.3, the heat transfer fluid ducts, and 3.4 and 4.4 the adsorbent tubes. The duct can have a circular, square, hexagonal or any other cross-section that is appropriate for a specific application. The external surfaces of the adsorbent tubes may have fins or other projections to enhance heat transfer as shown in FIG. 5. Longitudinal or spiral fins are preferred. In FIG. 5, components 5.1 and 5.4 are adsorbents, 5.2 and 5.5 are longitudinal and spiral fins respectively, and 5.3 and 5.6 are the adsorbent tubes. Heat transfer from the adsorbent to the walls of the internal wall of an adsorbent tube may advantageously be promoted by incorporating perforated metal plates, discs or other members composed of metal mesh or fibre which are disposed perpendicular to the tube axis within the adsorbent bed. For optimum heat transfer these should in close contact with the inner wall.

In another embodiment of this invention the heat transfer liquid flows through a tube within the absorption tube as shown in FIGS. 6 and 7. Component 6.1 is the adsorbent containing tube. When the adsorbent is under suction as shown in FIG. 6 valve 6.2 is open and valve 6.3 is closed so that desorbed working fluid leaves at 6.6. Heat transfer fluid flows into the central tube at 6.4 and leaves at 6.5. When the adsorbent is under compression 6.2 is closed and 6.3 is open. Simultaneously heat transfer fluid flows in at 6.5 and out at 6.4. FIG. 7 shows the adsorbent 7.1 surrounding the heat tube 7.3 containing the heat transfer fluid 7.2. Heat transfer between the adsorbent and the liquid tube may be enhanced by heat transfer fins which are either perpendicular to the axis of the tubes or are arranged in a spiral along the liquid tube. Preferably the fins fit closely or are attached to the outer surface of the liquid tube but are not in contact with the inner surface of the adsorbent tube. This is shown in FIG. 7 where 7.3 is in contact with heat transfer fin 7.5, shown partly cut away. 7.5 is perforated to allow the passage of working fluid. The adsorbent is contained within the outer tube 7.4. The heat transfer fluid tube can be circular in cross-section. Advantageously it can be pressed into an elliptical cross-section which retains its surface area but with reduced internal volume. This geometry provides a higher linear velocity for a given volume flow of heat transfer liquid resulting in an improved metal/liquid heat transfer coefficient.

In one embodiment of this invention the gas flow is advantageously constricted to generate a pressure gradient along the adsorbent tube. This can be achieved by in various ways, used alone or in various combinations. For example one method involves the use of solid heat exchanger fins in the form of unperforated discs perpendicular to the axes of the adsorbent and liquid tubes providing small gaps between their edges and the inner wall of the adsorbent tube through which the working fluid is constrained to pass. In a second method gaps between the fins and the inner wall are sealed by polymer gaskets, but the fins are perforated by small holes which restrict the gas flow. By varying the numbers of fins employed, the size of the gap between their edges and/or the diameters and number of the perforations the desired pressure gradient can be achieved.

In a further embodiment the inner wall of the adsorbent tube is provided with a low thermal conductivity liner. This may inhibit the flow of heat from the adsorbent to the wall of the adsorbent tube. This arrangement has the advantage that during the thermal cycling of the adsorbent the thermal capacity of the containing tube does not significantly reduce the magnitude of the temperature changes. The liner can also serve as container for the adsorbent and heat transfer liquid tube allowing them to be assembled prior to insertion in the adsorbent tube. A further advantage of this design is that adsorbent tube, which is not required for heat transfer, can be fabricated from materials such as mild or stainless steels which are inherently stronger than copper or aluminium, the metals generally favoured when high thermal conductivities are preferred. Also, apart from cost and weight, there is no constraint on the tube wall thickness selected which can thus be chosen to resist high pressure. This is especially advantageous when a multiplicity of heat transfer liquid pipes is employed as shown in FIG. 8. Adsorbent 8.1 is contained within tube 8.4 from which is isolated by insulating material 8.6. 8.2 is heat transfer fluid contained within tube 8.3. Heat transfer fin 8.5, shown partly cut away, enhances heat transfer from the adsorbent to the heat transfer fluid tubes. The liner is preferably fabricated from a low thermal conductivity organic polymer. More preferred is an open cell organic foam, such as polyurethane foam. Especially preferred is an organic foam reinforced externally with a solid polymer tube or surface layer to provide mechanical strength. In one embodiment of this invention the containing tube for the bed is fabricated from an engineering polymer such as polyether-ether-ketone, preferably reinforced with a material such as graphite fibre. Strong composites of this type are well known in the aerospace industry and are preferred where light weight construction is advantageous. In the context of this invention such materials are preferred when light weight is required, for example in vehicle cabin air conditioning devices. The tube carrying the heat transfer liquid is preferably fabricated from a metal to facilitate heat transfer. Preferably the tube is made from a high thermal conductivity metal such as copper of aluminium. A further advantage of this configuration is that the tube is exposed to an external pressure of gas rather than an internal pressure. In this mode copper and aluminium, which are less mechanically strong than steel, are acceptable.

In another embodiment of this invention the adsorption tube is fabricated in the form of a spiral and contained within the annulus of two concentric tubes that form a liquid duct as shown in FIGS. 9 and 10. Adsorbent 9.2 is contained within adsorbent tube 9.1. This design has the special advantage than it allows a long effective length of adsorption bed to be contained within much shorter actual length. In one embodiment the duct walls fit closely to the adsorption tube as shown in FIG. 10. Adsorbent 10.1 is contained within tube 10.2 which coiled around the inter duct wall 10.2 and enclosed by the outer duct wall 10.4. The heat transfer liquid is constrained to flow through the spiral passageways formed between the adsorption tube and the outer and inner ducts. This configuration minimises the quantity of heat transfer liquid in the heat exchanger and thus assists in keeping the temperature changes as large as possible. To reduce heat gain or loss to the immediate environment of the device the water duct can be advantageously insulated, for example with a layer of polystyrene or polyurethane foam or glass fibre, 10.5.

In a further embodiment of this invention the heat transfer fluid is a gas, preferably air. This may be caused to flow along the outer surface of the adsorbent tube. To provide good heat transfer the outer surface of the tube is fitted with longitudinal or spiral fins. Heat transfer from the adsorbent to internal wall of an adsorbent tube may be advantageously promoted by perforated metal plates or discs of metal mesh or fibre which are preferably located perpendicular to the tube axis. For optimum heat transfer these should in close contact with the inner wall. These discs can also serve to constrict the working fluid gas flow to generate a pressure gradient along adsorbent tube. For example if perforated metal plates are used the number of plates and the diameter and number of the perforations can be selected to give the desired pressure gradient.

Good heat transfer between the adsorbent and the heat transfer fluid is clearly desirable for optimum performance. Metal adsorbents which can be used with hydrogen are especially advantageous in that they have much higher thermal conductivities than non-metallic materials such as carbon, zeolites and silica gel.

The choice of the fluid/adsorbent combination may depend upon a number of factors whose values must be selected to provide an optimum performance for a given application and the design of the adsorbent heat exchangers. The adsorbent can be contained in tubes as described above. The adsorbent can be contained in sets of tubes in parallel, each set simultaneously undergoing compression or suction. Alternatively the adsorbent can be contained in sets of tubes in series connected by pipes. While the pressure drop across a single tube can be small a substantial pressure gradient can be established across the tube series by incorporating restrictions to the gas flow in the connecting pipes. In another embodiment of this invention the adsorbent is contained within a pair of plates sealed their edges and equipped with inlet and outlets at opposites ends of the plates. Heat transfer fluid removes or adds heat by flowing over the external faces of the plates. This mode of construction produces an adsorption plate heat exchanger. Sets of these plate heat exchangers can assembled in parallel into a module such that the heat transfer fluid flows between each pair of heat exchangers.

In a preferred embodiment of this invention a device comprises a working fluid, a positive displacement compressor driven by source of mechanical power, and a porous adsorbent solid through which pulses of compressed working fluid are able to expand. Cooling is produced by desorption of working fluid from the porous solid by reducing the pressure of the gas in contact with one end the solid while pressure induced adsorption of the working fluid by the solid at the other end produces heating. Preferred working fluid/adsorbent solid combinations are selected such that the heats of adsorption and desorption are substantially greater than the heat of compression. More preferably the heats of adsorption and desorption should be comparable with the latent heats of vaporisation of CFC, HCFC, HFC, hydrocarbon and ammonia working fluids presently used for conventional Rankine Cycle based devices which they are intended to replace.

One configuration for the device is shown in FIG. 11. The compressor consists of piston 11.1, moving in cylinder 11.2, driven by piston rod 11.3 attached to crank shaft 11.4, the latter being powered by an electric motor or other motive source, which is not shown. The compressor is fitted with two valves, 11.5 and 11.6. The inlet or suction valve 11.5 opens when the swept volume between the piston and the cylinder head is just below the pressure in heat exchanger 11.7. This occurs when the piston is moving towards bottom dead centre. Conversely the outlet or discharge valve 11.6 opens when the swept volume between the piston and the cylinder head is just above the pressure in heat exchanger 11.9. This occurs when the piston is moving towards top dead centre. Heat exchanger 11.8 rejects the heat of compression to atmosphere. Adsorbent porous solid 11.10 interacts with the working fluid reducing its pressure to a level for which the device is designed and providing a resistance to its flow so that a pressure difference can be maintained between 11.7 and 11.9. The device is charged with sufficient working fluid to maximise the heat pumping capacity but without compromising the pressure limitations of the design. Typically the device will be able to withstand maximum operating pressures of up to 30 barg. For lower cost devices the operating pressure will preferably not exceed 20 barg.

The device operates in a cycle described by the following steps starting from the state where the suction stroke of the reciprocating compressor has just been completed and the compression stroke is just about to start, i.e. bottom dead centre.

-   -   (a) The piston is driven into the cylinder simultaneously         raising the temperature and pressure of the gaseous working         fluid until the discharge valve 11.6 opens and compressed gas is         discharged into heat exchanger 11.8.     -   (b) Heat exchanger 11.8 rejects the heat of compression to an         air or water stream, or other appropriate heat sink.     -   (c) The cooled compressed gas then enters the high temperature,         porous solid-containing heat exchanger 11.9 where it is adsorbed         and the heat of adsorption rejected to an air stream which is         thereby heated.     -   (d) Working fluid travels though the porous solid towards the         low temperature heat exchanger 11.7 under the influence of the         pressure gradient established across the solid by the         compressor.     -   (e) The direction of the piston is reversed thus reducing the         pressure in the cylinder causing the suction valve 11.5 to open         so that working fluid is desorbs from the porous solid in the         low temperature heat exchanger 11.7. The heat of desorption is         supplied by an external air stream which is thereby cooled.     -   (f) The direction of the piston is again reversed, and starts to         compress the gas causing the suction valve to close thus         completing the cycle.

For the device to operate successfully in the mode described it is important that the pressure of the gas at any point in the porous solid connecting 11.7 and 11.9 oscillates about a mean value so that working fluid travels through the solid via a series of adsorptions and desorptions induced by the compressor. This process will provide the major contributions to the enthalpy changes in 11.7 and 11.9. To optimize the performance of the device the external air stream should also oscillate along the heat exchanger as shown in FIG. 11. These oscillations are phased such that during desorption of CO₂ by suction from a bed the air flows in the same direction as the CO₂. Conversely during adsorption of the CO₂ during compression the air flow is countercurrent to the CO₂ flow.

The reciprocating compressor shown in FIG. 11 could be replaced by any positive displacement compressor, including a rotary, sliding vane or diaphragm type. Compressors whose sliding surfaces in the displacement volume are lubricated by a liquid lubricant will require oil separators between the compressor and the adsorption bed to prevent oil in fine droplet form fouling the bed. Oil-free compressors are therefore preferred, i.e. compressors whose moving surfaces in contact with the working fluid are not lubricated in the displacement volume by a liquid lubricant. Especially preferred are diaphragm compressors which operate nearer to isothermal than isentropic conditions because of the effective cooling of the working fluid through the large surface area of the diaphragm and the compressor head augmented in some units by the cooling of the circulating hydraulic oil that drives the diaphragm. Diaphragm compressor energy efficiencies can be superior to those of reciprocating compressors. Fluid leakage rate is much lower because an excellent seal can be established between the diaphragm and compressor case, while an oil-free reciprocating compressor lacks the excellent sealing properties of the oil film of a conventional reciprocating unit. For low duty applications, such as domestic refrigerators and room air conditioning units, diaphragm compressors have the advantage over conventional oil-filled hermetic reciprocating and rotary units in the providing a combination of an excellent gas seal with an external electrical motor. The heat generated by the latter can be dissipated by a simple cooling fan. In conventional hermetic systems motor cooling is partly provided by the oil, which transfers heat to the casing, and the refrigerant, which transfers heat to the condenser. By removing requirement for internal cooling of the electric motor the energy efficiency of the cycle can be improved.

Various compressor designs can be used, provided they are configured to deliver pulses of compressed gas at the hot end of the adsorption bed and remove pulses of expanded gas at the cold end. FIG. 11 schematically illustrate one method for achieving this which is especially suited to adsorbents which have high thermal conductivities and thus good heat transfer from their bulk to the external gas stream. Hydrogen/porous metal adsorbent combinations in particular are suited for this purpose. The performance such combinations can be enhanced by including periodic thermal breaks in the adsorption bed, for example by including porous polymer plugs along periodically along the bed.

FIG. 12 schematically shows a further embodiment of this invention. A pressure difference is maintained between two vessels, 12.1 at low pressure and 12.2 at high pressure, by any type of compressor 12.3 capable of achieving the desired pressure ratio. This includes both pulsed positive displacement and continuous delivery turbo/centrifugal types, if necessary multi-staged. Pulses of compressed working fluid gas or vapour are delivered to the hot/high pressure heat exchanger 12.4 of the adsorbent bed 12.5 by periodically opening and closing powered valve 12.6. Pulses of expanded working fluid are removed from the cold/low pressure heat exchanger 12.7 by periodically opening and closing powered valve 12.8. The advantages of the design shown in FIG. 12 over that in FIG. 11 include the independent phasing of the compression and suction pulses and the ability to use continuous delivery compressors. Heat exchanger 12.9 rejects the heat of compression to atmosphere. The configuration shown in FIG. 12 allows the cycle time of pressure/suction pulses applied to the bed time to be significantly longer than the cycle time a positive displacement compressor used to power the system. This design is especially preferred when using adsorbents with lower thermal conductivities than porous metals.

Porous solids, such as activated carbons, have excellent adsorption capabilities for vapours and gases, but are poor thermal conductors requiring long cycle times, e.g. >1 minute, to enable heat to be taken in during the suction phase and heat to be rejected during the compression phase. If operated in this manner when the heat pump is being used for cooling it will only supply cold during half of its operating cycle. This limitation is overcome in a further preferred embodiment of this invention containing two beds operating 1800 out of phase, such that as one bed is under going suction/desorption while the other is undergoing compression/adsorption. This embodiment, employing a pair of adsorption tubes, is illustrated in FIGS. 13 a to 13 f. This device is intended to air condition an enclosed space such as a room or vehicle cabin. Cold air entering the enclosed space will typically be 10 to 15° C., while air exhausted to the outside environment will typically be 35 to 60° C. The adsorbent is contained in two finned tubes, 13.1 and 13.2 enclosed in two air ducts 13.12 and 13.13 shown in FIG. 13 b. Air is driven through the ducts by fans 13.5 and 13.9. Fan 13.9 sucks exhaust air from the room being air-conditioned while fan 13.5 pulls in external air. The flows from the two fans are periodically and simultaneously alternated between 13.12 and 13.13 by operating the movable vanes 13.14 and 13.15 shown in FIG. 13 c. In the latter the vanes are shown such that the external air flow from 13.5 is being blown into duct 13.13 while the exhaust room air is being directed through duct 13.12. The dotted lines indicate the alternative positions of the vanes. The working fluid is compressed by compressor 13.6 which is driven by motor 13.16. The flow of the working fluid through the equipment is controlled by the pressure equalisation valve 13.3 and switching valves 13.7 and 13.8 which serve periodically to switch the working fluid flow between 13.1 and 13.2, so that while one is under suction the other is under compression. The heat of compression is rejected via heat exchanger 13.10 over which air is driven by fan 13.11. FIGS. 13 d, 13 e, and 13 f, schematically showing this design with multiple adsorption tubes arranged in two sets, indicate its operation applied to a room air-conditioning device. The working fluid is driven around the circuit by compressor 13.6. To enhance heat transfer each bed consists of multiple parallel tubes packed with adsorbent in order to maximise the surface area exposed to the air stream. The operation of the cycle is described by means of example, but not in any limitative sense, by starting at point shown in FIG. 13 d. Bed 13.1 is at the room exhaust air temperature and contains working fluid adsorbed at the maximum operating pressure, for example 20° C. and 20 bar. 13.2 is at the temperature of the external ambient air and contains working fluid adsorbed at the minimum pressure in the system, for example at 30° C. and 1 bar.

-   -   a. With no air flow over either bed, with the compressor         switched off and with valves 13.7 and 13.8 closed valve 13.3 is         opened allowing working fluid to flow from 13.1 to 13.2.         Alternatively the working fluid can be arranged to pass through         the compressor or another engine to equalise the pressures with         the advantage that useful work may be obtained from the         compressor or engine. In the resulting adiabatic process the         temperature of 13.1 falls below the exhaust air temperature.         Conversely the temperature of 13.1 rises above the external         ambient temperature as fluid is adsorbed.     -   b. When the pressures have essentially equalised 13.3 is closed.         13.7 and 13.8 are opened and the compressor 13.6 is switched on         so that working fluid is pumped from 13.1 (desorption) to 13.2         (adsorption) (FIG. 13 e). Ambient external air flowing over 13.1         driven by fan 13.5 is thus cooled and enters the room at the         desired low temperature, for example 10° C. Conversely an equal         volume of exhaust room air is removed from the room by fan 13.9         and flows over 13.2 where it is heated to a temperature above         that of the external ambient air before being rejected to         atmosphere.     -   c. Because 13.1 and 13.2 are designed to have lengths         substantially greater than their diameters the temperatures of         the air streams exiting from each adsorbent containing heat         exchanger will remain approximately constant until 13.1 has         essentially been heated to the ambient air temperature along the         whole of its length, while 13.2 has essentially been cooled to         the temperature of the exhaust air.     -   d. When this point has been reached the device will have reached         a condition similar to the initial condition (a) but with 13.1         now at low pressure and ambient temperature and with 13.2 at         high pressure and exhaust air temperature. In other words 13.1         and 13.2 have reversed their roles.     -   e. The cycle continues as described in (a) to (c) above until         the device returns to its initial condition. To achieve this         result vanes 13.14 and 13.15 are switched thus reversing the air         flows through the ducts 13.12 and 13.13 in FIG. 13 f.         Simultaneously valves 13.7 and 13.8 are re-set allowing the         compressor to transfer the working fluid from 13.2 to 13.1.

In a further preferred embodiment of the device more than one pair of adsorbent heat exchangers are used so that when the pressure is being equalized between one pair of heat exchangers the compressor continues moving working fluid between the two members of a second pair of heat exchangers. This arrangement has the advantage of providing effective continuous heat pumping.

In a further embodiment of this invention the adsorbent beds are cooled and heated by a circulating liquid. FIG. 14 illustrates one possible design for this device. The adsorbent tubes containing internal heat transfer tubes are in a circuit with compressor 14.7, pressure equalisation valve 14.11, flow switching valves 14.3 and 14.4, and heat exchanger 14.12 which rejects the heat of compression. The liquid circuit incorporates eight fluid logic diodes, arranged in two bridge rectifiers 14.5 and 14.6, a liquid pump 14.10 whose input and output flows can be reversed periodically, and two external heat exchangers 14.8 and 14.9. This design allows liquid to pass through the external heat exchangers 14.8 and 14.9 in the same direction while allowing the liquid to oscillate in the adsorbent heat exchangers. The heat transfer liquid circuit is shown by solid lines in FIG. 14. The cycle employed is essentially that described for the device in FIGS. 13 d, 13 e, and 13 f. The working fluid is desorbed from one bed and compressed on to the other by the action of compressor 14.7. The dotted lines show the working fluid circuit. In one embodiment of this invention the liquid passes through one or more tubes or pipes within the adsorbent bed. Alternatively the liquid may flow through a duct external to the adsorption containing tube. The device represented in FIG. 14 is suited to a refrigerated enclosure, and to a water chiller air conditioning system where significant proportion of the air is recycled within the room or building.

A suitable heat transfer liquid requires a combination of properties which are determined by its intended application. For air conditioning air is an attractive option. When liquids are employed they preferably have low viscosities to minimise pumping energy. Preferably liquids should have dynamic viscosities less than 0.025 Pa·s, preferably less than 0.01 Pa·s, and most preferably less than 0.001 Pa·s. Provided the liquid circulation system is suitably pressurized, liquids with a range of boiling points can be considered. Preferably for operating convenience the liquid should have a normal boiling point greater than highest temperature reached by the adsorbent. The liquid must not freeze below the lowest temperature generated within the device. Preferably the liquid has a flash point greater that 100° C., more preferably greater than 130° C. and even more preferably greater than 200° C. Most preferably the liquid should be non-flammable. Preferred liquids include those already known to the industry as secondary refrigerants. These materials include water, brines, glycols, alcohols, hydrocarbon oils, silicone oils, and halogenated compounds including partially fluorinated ethers, perfluorinated ethers and chlorinated liquids. Where they are mutually compatible these liquids may also be used in mixtures.

For low refrigeration temperatures down to −50° C. compositions with wide liquid ranges are require, while retaining the desirable properties of flash points greater than 100° C. and normal boiling points greater than the highest temperature. Preferred substances include esters and ethers containing 3 or more carbon atoms which can be acyclic or cyclic. Preferred substances include, but are not limited, to glycol- or polyol-cyclic carbonates and cyclic ethers. Especially preferred are propylene carbonate, ethylene carbonate and dimethylisosorbide. Blends comprising esters, ethers, glycols with each other and with water can also be used. The liquids may optionally contain additives which enhance one or more desirable composition properties such as lower freezing points, higher boiling points, lower viscosities or higher flash points. If such additives were to be used alone they would not be preferred, but are acceptable when used in mixtures where they constitute less than 50% of composition by mass.

To avoid adverse environmental impacts compositions containing fluorine or chlorinated substances these compounds should preferably have very low vapour pressures or incorporate reactive groups such as double or triple bonds that facilitate their rapid destruction by reactive species in troposphere. 

1. A heat pump device comprising: at least one heat exchanger; a body of porous absorbent material housing an inlet and an outlet the body being disposed in thermal contact with the heat exchanger, means for passing a working fluid through the body, and means for inducing cyclical compression and expansion pulses in the working fluid, to cause the working fluid to flow from the inlet to the outlet to create a temperature gradient in the body between the inlet and outlet.
 2. A device as claimed in claim 1, further including a heat transfer fluid, means for passing the heat transfer fluid in thermal contact with the heat exchanger, arranged so that the flow of heat transfer fluid removes or adds heat from or to the heat exchanger.
 3. A device as claimed in claim 2, wherein the direction of flow of the heat transfer fluid changes with the compression and expansion pulses of the working fluid.
 4. A device as claimed in claim 3, wherein the direction of flow of the heat transfer fluid reverses in concert with the compression and expansion pulses of the working fluid.
 5. A device as claimed in claim 4, wherein the reversal of direction of flow of the heat transfer fluid is synchronized with said pulses.
 6. A device as claimed in claim 3, wherein the frequency of the cyclical motion of the working fluid is the same as the frequency of cyclical motion of the heat transfer fluid.
 7. A device as claimed in claim 1, wherein the means for inducing pulses in the working fluid is a positive displacement compressor.
 8. A device as claimed in claim 1, wherein the means for inducing pulses in the working fluid comprises a valve switching system and a compressor.
 9. A device as claimed in claim 8, wherein the valve switching system alternatively connects the body of absorbent material to high and low pressure reservoirs of the working fluid.
 10. A device as claimed in claim 1 comprising a further heat exchanger adapted to remove heat of compression from the working fluid before contacting the absorbent material.
 11. A device as claimed in claim 1, wherein the temperature gradient comprises a relatively high temperature at the inlet and a relatively low temperature at the outlet.
 12. A device as claimed in claim 1, wherein the working fluid is selected from the group consisting of a vapour or gas or a mixture thereof.
 13. A device as claimed in claim 1 comprising a plurality of heat exchangers.
 14. A device as claimed in claim 1 in which the working fluid is a single fluorocarbon or a mixture of fluorocarbons boiling between −140° C. and 40° C.
 15. A device as claimed in claim 14, wherein the working fluid has a boiling point between −90° C. and 0° C.
 16. A device as claimed in claim 15, wherein the working fluid has a boiling point between −90° C. and −20° C.
 17. A device as claimed in claim 1 in which the working fluid is a hydrocarbon selected from the group consisting of methane, ethane, propane, iso-butane and butane, and mixtures thereof.
 18. A device as claimed in claim 1 in which the working fluid is nitrogen.
 19. A device as claimed in claim 1 in which the working fluid is carbon dioxide.
 20. A device as claimed in claim 1 in which the working fluid is hydrogen.
 21. A device as claimed in claim 1 in which the working fluid is a noble gas.
 22. A device as claimed in claim 1 in which the porous solid has at least 10% of its void volume in the form of micropores with diameters less than 2 nm.
 23. A device as claimed in claim 22 in which the porous solid has at least 10% of its void volume in the form of mesopores with diameters less than 50 nm.
 24. A device as claimed in claims 23 in which the porous solid has less than 20% of its void volume in the form of macropores with diameters greater than 50 nm.
 25. A device as claimed in claim 1 in which the adsorbent porous solid is a carbon-based material.
 26. A device as claimed in claim 25 in which the adsorbent porous solid is a charcoal material.
 27. A device as claimed in claim 26 in which the adsorbent porous solid is an activated carbon material.
 28. A device as claimed in claim 25 in which the adsorbent porous solid is an organic polymer-based material.
 29. A device as claimed in claim 25 in which the adsorbent porous solid is an essentially inorganic material.
 30. A device as claimed in claim 29 in which the adsorbent porous solid is the oxide of a metal or metalloid element, or combination thereof.
 31. A device as claimed in claim 29 in which the adsorbent porous solid is a zeolite.
 32. A device as claimed in claim 29 in which the adsorbent porous solid is a sieve.
 33. A device as claimed in claim 29 in which the adsorbent porous solid is selected from the group consisting of silica, alumina, titanium dioxide and mixtures thereof.
 34. A device as claimed in claim 1 in which the adsorbent porous solid is an aerogel.
 35. A device as claimed in claim 1 in which the adsorbent porous solid is impregnated with a low volatility solvent.
 36. A device as claimed in claim 1 in which the adsorbent porous solid includes an additive to enhance thermal conductivity.
 37. A device as claimed in claim 1 in which the enthalpies of adsorption and desorption in the heat exchanger are essentially the same.
 38. A device as claimed claim 1 wherein said pulses have duration in the range of 1 second to 10 minutes.
 39. A device as claimed in claim 36, wherein the duration is in the range of 1 second to 30 seconds.
 40. A device as claimed in claim 39, wherein the duration is in the range of 1 second to 10 seconds.
 41. A device as claimed in claim 1 wherein there is a pressure gradient of working fluid between the inlet and outlet.
 42. A device as claimed in claim 1 wherein the working fluid is a mixture.
 43. A device as claimed in claim 42, wherein the working fluid is a mixture of a strongly adsorbed fluid and weakly adsorbed fluid.
 44. A device as claimed in claim 42 wherein the working fluid is a mixture of carbon dioxide and nitrogen.
 45. A device as claimed in claim 42 wherein the working fluid is a mixture of carbon dioxide and argon.
 46. A device as claimed in claim 42 wherein the working fluid is a mixture of carbon dioxide or ammonia and hydrogen, helium or a mixture thereof.
 47. A device as claimed in claim 42 wherein the working fluid is a mixture of carbon dioxide and propane.
 48. A heat pump device in which a temperature difference is established between two heat exchangers by inducing cyclical expansion and compression pulses in a working fluid vapour or gas which passes through an adsorbent porous solid located between the heat exchangers.
 49. A device as claimed in claim 39 in which the temperature difference between each heat exchanger and an external, single-phase, heat transfer liquid is essentially constant. 